Perpendicular magnetic recording medium and method of manufacturing the same and product thereof

ABSTRACT

The quantity of oxide contained in a magnetic layer is controlled to control the crystal grains and the segregation structure for ensuring low noise characteristic in a granular magnetic layer of a perpendicular magnetic recording medium. The granular magnetic layer consists of ferromagnetic crystal grains and a nonmagnetic grain boundary region mainly of an oxide surrounding the ferromagnetic crystal grains. The perpendicular magnetic recording medium has a nonmagnetic underlayer composed of a metal or alloy having hexagonal closest-packed crystal structure. The ferromagnetic crystal grain is composed of an alloy containing at least cobalt and platinum. The volume proportion of the nonmagnetic grain boundary region mainly of the oxide falls within a range of 15% to 40% of the volume of the total magnetic layer.

This is a divisional of U.S. patent application Ser. No. 11/468,905filed Aug. 31, 2006, which is a continuation of U.S. patent applicationSer. No. 10/314,907, filed Dec. 9, 2002, now U.S. Pat. No. 7,147,942.The contents of each are incorporated herein by reference.

BACKGROUND

High density magnetic recording can be attained with a perpendicularmagnetic recording system, as an alternative to a conventionallongitudinal magnetic recording system. In this regard, a crystallinefilm of a CoCr alloy system has been mainly contemplated for a magneticrecording layer of a perpendicular magnetic recording medium. Inperpendicular magnetic recording, the crystal orientation of therecording layer is controlled so that the c-axis of the CoCr alloysystem having a hcp structure aligns perpendicular to the film surface(i.e., the c-plane is parallel to the film surface). To obtain a higherdensity in the CoCr alloy system, finer grain size, reduction ofdispersion of grain size distribution, and decrease in magneticinteraction between grains have been contemplated.

A method of controlling the magnetic recording layer structure to raisethe recording density in a longitudinal recording medium has beenproposed, for example, in U.S. Pat. No. 5,679,473. A magnetic layer inthis reference, generally referred to as a granular magnetic layer, hasa structure in which magnetic crystal grains are surrounded by anonmagnetic, nonmetallic substance, such as an oxide or nitride. Becausethe nonmagnetic and nonmetallic grain boundary phase physicallyseparates the magnetic grains or particles in the granular magneticfilm, the magnetic interaction between the magnetic particles decreasesto suppress the formation of a zigzag shaped magnetic domain wall thatwould be formed in a transition region of a recording bit. Thus, lownoise characteristic can be achieved.

In the same vein, a granular magnetic layer is contemplated for arecording layer of a perpendicular magnetic recording medium in IEEETrans. Mag., Vol. 36, p 2393 (2000). Specifically, this publicationdiscloses a perpendicular magnetic recording medium comprising aruthenium metal underlayer and a magnetic recording layer of a CoPtCrOalloy having a granular structure. A magnetic film having the granularstructure is formed by reactive sputtering in an oxygen-containingatmosphere using a CoPtCr target. However, since the quantity of thegenerated oxide is extremely sensitive to the oxygen content of thesputtering atmosphere, it is difficult to control the quantity of theoxide formation surrounding the magnetic crystal grains. Moreover,because the magnetic crystal grain is easily oxidized, separating thematerial composing the magnetic crystal grains and the materialcomposing the oxide grain boundary is extremely difficult.

Accordingly, there is a need to control the quantity of oxide containedin the granular magnetic layer to control the crystal grains and thesegregation structure for ensuring a low noise characteristic. There isalso a need to form a superior magnetic characteristic by removing theoxide from the magnetic crystal grain. The present invention addressesthese needs.

SUMMARY OF THE INVENTION

The present invention relates to a perpendicular magnetic recordingmedium and a manufacturing method thereof. Such a perpendicular magneticrecording medium can be mounted on various magnetic recording devicessuch as an external memory device of a computer.

One aspect of the present invention is a perpendicular magneticrecording medium having a nonmagnetic substrate and at least thefollowing layers sequentially laminated on the substrate: a nonmagneticunderlayer, a magnetic layer, and a protective film. The nonmagneticunderlayer can be composed of a metal or an alloy having a hexagonalclosest-packed (hcp) crystal structure. The magnetic layer can consistof ferromagnetic crystal grains and nonmagnetic grain boundary regioncomposed mainly of an oxide surrounding the grains. The ferromagneticcrystal grain can be composed of an alloy containing at least cobalt andplatinum. The volume of the nonmagnetic grain boundary region can fallwithin a range of 15% to 40% of the total volume of the magnetic layer.The thickness of the magnetic layer can fall within a range of 5 nm to20 nm. Moreover, the nonmagnetic substrate can be made of a plasticresin.

Another aspect of the present invention is a method of manufacturing theperpendicular magnetic recording medium described above. The methodfeatures depositing the nonmagnetic underlayer on the nonmagneticsubstrate, depositing the magnetic layer on the underlayer by RFmagnetron sputtering using a composite target containing a ferromagneticalloy and an oxide, and depositing the protective film on the magneticlayer. The depositing steps of the nonmagnetic underlayer, the magneticlayer, and the protective film can be performed without preheating thenonmagnetic substrate. The composite target can contain an oxide at avolume proportion falling within a range of 20% to 35% of the totalvolume of the target.

Another aspect of the present invention is a product formed by the abovemethod.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a structure ofan embodiment of a perpendicular magnetic recording medium according tothe present invention.

FIG. 2 is a graph showing the relationship between the coercive force Hcand the volume proportion of the nonmagnetic grain boundary region inthe magnetic layer.

FIG. 3 is a graph showing the relationship between the volume proportionof SiO₂ contained in the target and the volume proportion of thenonmagnetic grain boundary region in the magnetic layer.

FIG. 4 is a graph showing the relationship between the Hc value and thethickness of the magnetic layer.

DETAILED DESCRIPTION

The present inventors made extensive studies on the quantity of theoxide contained in the magnetic layer and finding a way of separatingthe oxide to the grain boundary in a perpendicular magnetic recordingmedium using a granular magnetic layer. The present inventors havediscovered that the following structure and manufacturing method canresult in a superior perpendicular magnetic recording medium by using agranular magnetic layer consisting of ferromagnetic crystal grains andnonmagnetic grain boundary region mainly composed of an oxidesurrounding the grains.

FIG. 1 is a schematic cross-sectional view illustrating a structure ofan embodiment of a perpendicular magnetic recording medium according tothe present invention. The perpendicular magnetic recording mediumcomprises a nonmagnetic substrate 1 and at least the following layerssequentially laminated on the substrate: a nonmagnetic underlayer 2, amagnetic layer 3, and a protective film 4. A liquid lubricant layer 5can formed on this laminate. A seed layer 11 can be provided between thenonmagnetic underlayer 2 and the nonmagnetic substrate 1 for the purposeof controlling the crystal alignment and grain size of the nonmagneticunderlayer 2. A relatively thick soft magnetic layer generally referredto as a backing layer (not illustrated) having a thickness of severalhundred nanometers can also be provided between the nonmagneticunderlayer 2 and the nonmagnetic substrate 1 for the purpose ofenhancing regeneration sensitivity.

The nonmagnetic substrate 1 can be made of NiP-plated aluminum alloy,strengthened glass, or crystallized glass, which are all conventionallyused in a magnetic recording medium. Because substrate heating isunnecessary, the substrate also can be an injection moldedpolycarbonate, polyolefin, or other plastic resin.

The nonmagnetic underlayer 2 is composed of metal or alloy having ahexagonal closest-packed (hcp) crystal structure for controlling crystalorientation in the granular magnetic layer. Such materials include ametal selected from Ti, Re, Ru, and Os, and an alloy containing at leastone element selected from Ti, Re, Ru, and Os. The thickness of theunderlayer can be in the range of 5 nm to 30 nm.

When the seed layer 11 is provided for the purpose of controlling thecrystal orientation and the grain size of the nonmagnetic underlayer 2,the seed layer can be composed of a metal or alloy having a facecentered cubic (fcc) crystal structure. Such materials include a metalselected from Cu, Au, Pd, Pt, and Ir, an alloy containing at least oneelement selected from Cu, Au, Pd, Pt, and Ir, and an alloy containing atleast Ni and Fe.

A thin film composed mainly of carbon, for example, can be used as theprotective film 4. A perfluoropolyether lubricant, for example, can beused as the liquid lubricant layer 5.

The magnetic layer 3 is a so-called granular magnetic layer consistingof ferromagnetic crystal grains and nonmagnetic grain boundary regionsurrounding the grains. The grain boundary region is composed of a metaloxide. The ferromagnetic crystal grain can be that of an alloycontaining at least Co and Pt. Elements such as Cr, Ta, B, or Cu can beadded to the CoPt alloy to control the magnetic characteristic andattain a low noise characteristic. The ferromagnetic crystal grain has astructure in which the c-axis of the crystal lattice is predominantlyaligned perpendicular to the film surface. The volume of the nonmagneticgrain boundary region is in the range of 15% to 40% of the entire volumeof the magnetic layer. When the volume is under 15%, a sufficient amountof grain boundary cannot exist between the grains, and the magneticinteraction between the grains cannot be decreased effectively. When thevolume is over 40%, the crystal alignment of the grains deteriorates.

The oxide for forming the grain boundary is not limited to specialmaterials to the extent that the oxide used is physically and chemicallystable. The oxide can be an oxide of Mg, Cr, Ti, Zr, or Si.

The thickness of the magnetic layer 3 can be in the range of 5 nm to 20nm. A thickness below 5 nm is not desirable because the magnetic layercannot output sufficient regeneration signals. A thickness over 20 nm isalso not desirable because the grain size is liable to expand and thecrystal alignment is apt to become disordered.

In producing a magnetic recording medium having the above-describedlayer construction and illustrated in FIG. 1, the magnetic layer 3 canbe formed by the following manufacturing method.

The magnetic layer is preferably deposited by means of an RF magnetronsputtering technique or method, using a composite target containing aferromagnetic alloy and an oxide. By using this technique, separationbetween the crystal grain and the nonmagnetic grain boundary is promotedas compared with the case of reactive sputtering in an oxygen-containingatmosphere using a target not containing an oxide. Thus, a favorablegranular structure can be obtained by the present method.

In addition, the volume of the oxide contained in the composite targetis directed in the range of 20% to 35% of the volume of the entiretarget to control the volume of the nonmagnetic grain boundary formed inthe magnetic layer within the desirable range indicated above.

In the process of manufacturing a perpendicular magnetic recordingmedium, even though it omits substrate heating, which is essential in aconventional magnetic recording medium, the present method stillproduces an excellent perpendicular magnetic recording medium. By virtueof the simplification of the production process, production costs can bereduced. Because substrate heating is unnecessary, the substrate can bemade of plastic, such as polycarbonate or polyolefin.

The following describes specific examples of preferred embodiments ofthe present invention. These examples are presented only for an aid toappropriately explain the invention and not for restricting theinvention.

In the first example, the nonmagnetic substrate 1 used was a 3.5 inchdisk of injection-molded polycarbonate. After cleaning, the substratewas introduced into a sputtering apparatus. A seed layer 11 of platinumhaving a thickness of 5 nm was formed on the substrate 1 under argon gaspressure of 5 mTorr. Then, a nonmagnetic underlayer 2 of rutheniumhaving a thickness of 20 nm was formed over the seed layer 11 underargon gas pressure of 5 mTorr. Subsequently, a granular magnetic layer 3having a thickness of 20 nm was formed by an RF magnetron sputteringtechnique or method using a Co₇₅Cr₁₀Pt₁₅ target containing variousquantities of SiO₂ under argon gas pressure of 5 mTorr. After laminatinga carbon protective film 4 having a thickness of 10 nm on the magneticlayer, the resulting medium was taken out from the vacuum chamber of thesputtering apparatus. A liquid lubricant layer 5 having a thickness of1.5 nm was formed over the protective film 4, a magnetic recordingmedium having the structure as shown in FIG. 1 was produced. Substrateheating prior to the deposition of the laminating layers was notperformed.

FIG. 2 shows the relationship between the coercive force Hc and thevolume proportion of the nonmagnetic grain boundary region in themagnetic layer. The coercive force was measured by a vibrating samplemagnetometer applying magnetic field perpendicular to the film surface.It can be seen that the Hc values higher than 3,000 Oe can be obtainedwhen the volume proportion of the nonmagnetic grain boundary region inthe magnetic layer is in the range of 15% to 40%.

FIG. 3 shows the relationship between the volume proportion of SiO₂contained in the target and the volume proportion of the nonmagneticgrain boundary region in the magnetic layer. The volume proportionoccupied by the nonmagnetic grain boundary region in the magnetic layerwas obtained by measuring the area occupied by grain boundary region andthe area occupied by grains on a planar image taken by a transmissionelectron microscope (TEM). FIG. 3 indicates that the volume proportionof the nonmagnetic grain boundary region in the magnetic layer is in therange of 15% to 40% when the volume proportion of the SiO₂ in the targetis in the range of 20% to 35%.

In the second example, a magnetic recording medium having a structure asshown in FIG. 1 was produced in the same manner as in the first exampleexcept that the quantity of SiO₂ contained in the target was fixed at25% and the thickness of the deposited magnetic layer was varied from 3nm to 50 nm.

FIG. 4 shows the relationship between the Hc values and the thickness ofthe magnetic layer. Hc was measured in the same manner as describedrelating to FIG. 2. Hc values higher than 3,000 Oe were obtained by themagnetic layer with the thickness in the range 5 nm to 20 nm. It can beextrapolated that Hc decreases in the region thinner than 5 nm by theinfluence of thermal disturbance, while in the region thicker than 20nm, Hc decreased due to the disordered crystal alignment in the grainsin the magnetic layer.

A perpendicular magnetic recording medium of the present inventioncomprises a granular magnetic layer that is a magnetic layer consistingof ferromagnetic crystal grains and a nonmagnetic grain boundary regioncomposed mainly of an oxide surrounding the grain, and a nonmagneticunderlayer composed of a metal or alloy having a hexagonalclosest-packed (hcp) crystal structure. The ferromagnetic crystal grainis composed of an alloy containing at least cobalt and platinum. Bycontrolling the volume of the nonmagnetic grain boundary region withinthe range of 15% to 40% of the volume of the whole magnetic layer, thecrystal grains and the grain boundary region are favorably controlled,to achieve excellent magnetic and low noise characteristics.

In addition, by controlling the thickness of the magnetic layer in therange from 5 nm to 20 nm, necessary and sufficient regeneration outputcan be gained; and crystal alignment is prevented from deteriorating,and the grain size is prevented from enlarging.

In a method of manufacturing a perpendicular magnetic recording mediumaccording to the invention, a magnetic layer is deposited by an RFmagnetron sputtering method using a composite target containing aferromagnetic alloy and an oxide. By controlling the volume of the oxidecontained in the target in the range of 20% to 35% of the volume of thetotal target, the oxide entrapped in the ferromagnetic crystal grainscan be minimized and the quantity of the oxide mainly composing thegrain boundary can be controlled within a preferable range.

By employing the structure of a medium and the manufacturing method asdescribed above, an excellent perpendicular magnetic recording mediumcan be obtained, even without substrate heating prior to the depositingprocesses. As a result, simplification and cost reduction of theproduction process can be achieved. Further, inexpensive plastics can beused for a substrate, as well as a conventional aluminum substrate and aglass substrate.

Given the disclosure of the present invention, one versed in the artwould appreciate that there may be other embodiments and modificationswithin the scope and spirit of the present invention. Accordingly, allmodifications and equivalents attainable by one versed in the art fromthe present disclosure within the scope and spirit of the presentinvention are to be included as further embodiments of the presentinvention. The scope of the present invention accordingly is to bedefined as set forth in the appended claims.

The disclosure of the priority application, JP PA 2001-374897, in itsentirety, including the drawings, claims, and the specification thereof,is incorporated herein by reference.

1. A method of manufacturing a perpendicular magnetic recording medium,comprising steps of: depositing a nonmagnetic underlayer composed of ametal or an alloy having a hexagonal closest-packed (hcp) crystalstructure on a nonmagnetic substrate; depositing a magnetic layerconsisting essentially of ferromagnetic crystal grains and a nonmagneticgrain boundary region surrounding the ferromagnetic crystal grains bysputtering using a composite target containing a ferromagnetic alloy andan oxide on the underlayer, wherein the ferromagnetic crystal grains arecomposed of an alloy containing at least cobalt and platinum, andwherein a volume occupied by the oxide contained in the composite targetfalls within a range of 20% to 35% of a total volume of the target.
 2. Amethod of manufacturing a perpendicular magnetic recording mediumaccording to claim 1, wherein the oxide in the target is an oxide of Mg,Cr, Ti, Zr, or Si.
 3. A method of manufacturing a perpendicular magneticrecording medium according to claim 1, wherein the oxide in the targetis SiO₂.
 4. A method of manufacturing a perpendicular magnetic recordingmedium according to claim 2, wherein the oxide in the magnetic layer isan oxide of Mg, Cr, Ti, Zr, or Si.
 5. A method of manufacturing aperpendicular magnetic recording medium according to claim 3, whereinthe oxide in the magnetic layer is SiO₂.
 6. A method of manufacturing aperpendicular magnetic recording medium according to claim 1, wherein avolume of the grain boundary region falls within a range of 15% to 40%of a total volume of the magnetic layer.
 7. A method of manufacturing aperpendicular magnetic recording medium according to claim 4, wherein avolume of the grain boundary region falls within a range of 15% to 40%of a total volume of the magnetic layer.
 8. A method of manufacturing aperpendicular magnetic recording medium according to claim 5, wherein avolume of the grain boundary region falls within a range of 15% to 40%of a total volume of the magnetic layer.